Although silicon is an excellent material for guiding light and can detect and modulate light at high data rates, it has not yet been capable of efficiently generating large amounts of light. Silicon may be an inefficient light emitter because of a fundamental limitation called an indirect bandgap. An indirect bandgap prevents the atoms in silicon from emitting photons in large numbers when an electrical charge is applied. Instead, silicon tends to emit heat.
One approach to generate light on a silicon material is the so-called hybrid silicon evanescent platform. Indium Phosphide is one of a few special materials, including Gallium Arsenide, which emits energy as a photon of light when voltage is applied. Both materials may be used to make laser diodes, and are referred to as ‘III-V materials’ on the Periodic Table of elements because they share similar characteristics.
The hybrid silicon evanescent platform approach involves using wafer bonding to fix in place a III-V semiconductor material capable of emitting light above a silicon-on-insulator (SOI) wafer which is used for guiding the light. With the proper silicon waveguide design, light that is generated in the III-V material by current injection is automatically coupled into the silicon waveguide and laser light is efficiently coupled to the silicon waveguides. Several examples of these lasers have been demonstrated and their merits are now well-accepted.
However, there are some inherent disadvantages of these lasers due to their in-plane structure. To date, in-plane lasers have not demonstrated the high wall-plug efficiencies (i.e. overall energy efficiency) of vertical cavity surface emitting lasers (VCSELs). Vertical cavity lasers also have inherently higher direct modulation bandwidths with lower modulation powers. The advantages of the VCSEL result from the very low threshold currents that are possible for vertical cavity lasers, and this may be due to a combination of effects including the short cavity length and high electric field overlap with the quantum well (gain) material. If VCSELs are used, there are other problems such as alignment issues. Current methods involve die bonding individual VCSELs above silicon or coupling lasers in from the edge of a silicon chip. Both methods are time consuming and require sensitive alignment techniques.
FIG. 1 is a graph showing simulated and experimental threshold current for VCSELs with different reflectivity products for the front and back mirrors. Although results may also vary depending on the exact material used to form the laser active section, it is clear that very low thresholds can be attained, and this has been demonstrated experimentally. Thresholds below 1 mA are routinely obtained today.